How do I call php script each time html file loads?

If it's a PHP page, you can include/require a page in to another PHP page using require or include see include and require documentation

require("counting_script.php");
// or
include("counting_script.php");

If it's a purely HTML file, then you could consider using an iFrame to load your other page see tutorial

<iframe src="page_counter.php"></iframe>

Or if you know jQuery, you could make an ajax call (using jQuery) to your counting page. See documentation

$.get('page_counter.php', function(data) {
  alert('Counter page was called');
});

Some webpages uses a transparent 1x1 pixel image that can count number of requests, it works by letting PHP return a image. This is the code for image.php:

header('Content-type: image/gif');
echo chr(71).chr(73).chr(70).chr(56).chr(57).chr(97).chr(1).chr(0).chr(1).chr(0).chr(128).chr(0).chr(0).chr(0).chr(0).chr(0).chr(0).chr(0).chr(0).chr(33).chr(249).chr(4).chr(1).chr(0).chr(0).chr(0).chr(0).chr(44).chr(0).chr(0).chr(0).chr(0).chr(1).chr(0).chr(1).chr(0).chr(0).chr(2).chr(2).chr(68).chr(1).chr(0).chr(59);

So now you simply create the correct HTML for it like this:

<img src="image.php"/>

Notice that you also can send variables and other useful information to your PHP-file with arguments like:

<img src="image.php?username=test"/>

You also recieve a lot of other useful information in this approach like being able to use the $_SERVER-variables to determine what URL the user saw when the image.php file was requested, so now you are able to make per-page statistics.

Another workaround is to use an img tag

<img src="page_counter.php" alt="counter"/>

and inside your page_counter.php file

<?php 
//your php counter script

//return an 1x1 transparent image
$image = file_get_contents('your_path/1x1.png'); 
header('content-type: image/png'); 
echo $image;
?>